1. Field of the Invention
The present invention relates to a semiconductor device having an organic semiconductor.
2. Description of the Related Art
The development of a thin film transistor using an organic semiconductor has gradually become active since the latter half of the 1980s. In addition, the basic performance of a thin film transistor using an organic semiconductor has recently exceeded the basic performance of a thin film transistor using amorphous silicon. An organic material can be easily processed in many cases, and often has a high affinity for a flexible plastic substrate on which a thin film field effect transistor (FET) is formed. Therefore, the organic material is an attractive material for a semiconductor layer in a device requested to have flexibility or a light weight. The following organic semiconductor materials have been heretofore investigated. A first example is an acene-based compound such as pentacene or tetracene disclosed in Japanese Patent Application Laid-Open No. H05-55568. A second example is any one of phthalocyanines each containing lead phthalocyanine disclosed in Japanese Patent Application Laid-Open No. H05-190877. A third example is a low-molecular weight compound such as perylene or a tetracarboxylic acid derivative of perylene. A fourth example is any one of: aromatic oligomers typified by a thiophene hexamer referred to as α-thienyl or sexithiophene disclosed in Japanese Patent Application Laid-Open No. H08-264805; and polymer compounds such as polythiophene, polythienylenevinylene, and poly-p-phenylenevinylene. It should be noted that most of those materials are described in Advanced Material, 2002, no. 2, p. 99 to 117.
Properties requested for producing a device using any one of those compounds in its semiconductor layer such as non-linear optical property, conductivity, and semiconductivity largely depend on not only the purity of a material for the device but also the crystallinity and orientation of the material. An organic material showing good semiconductor property is generally a compound with an expanded π conjugated system. Meanwhile, a compound with an expanded π conjugated system is generally insoluble or hardly soluble in a solvent. For example, pentacene has high crystallinity and is insoluble in a solvent, so a pentacene thin film is generally formed by employing a vacuum vapor deposition method. As a result, a pentacene thin film showing a high field effect mobility is obtained. However, when the vacuum vapor deposition method is employed, the good processability of an organic material is not sufficiently exerted because of, for example, the large size of an apparatus for the method and a long time period needed for the production of a film.
Meanwhile, there has been also reported an FET using a film obtained by: forming a thin film of a soluble precursor for pentacene through application; and transforming the precursor into pentacene through a heat treatment (see US 2003/0136964 A1).
Further, it has been reported that tetrabenzoporphyrin obtained by heating porphyrin, in which a bulky bicyclo[2.2.2]octadiene skeleton undergoes ring condensation, at 210° C. or higher can be used as an organic semiconductor (speech proceedings II of the 81st spring annual meeting of the Chemical Society of Japan, 2002, p. 990 (2F 9-14), Japanese Patent Application Laid-Open No. 2003-304014, and Japanese Patent Application Laid-Open No. 2004-6750). Although the field effect mobility of an organic semiconductor layer described in any one of those documents is high, the high field effect mobility is realized merely on a silicon substrate or a glass substrate identical to amorphous silicon or the like. Meanwhile, the formation of an organic semiconductor layer stably showing a high mobility even on a resin substrate plays an important role in the realization of a flexible device taking advantage of the characteristics of an organic material.
A known method of forming an organic semiconductor layer stably showing a high mobility is a method involving controlling an interface between a gate insulating layer and the organic semiconductor layer. For example, in Japanese Patent Application Laid-Open No. 2005-32774, the threshold voltage of an organic semiconductor layer to be formed on a gate insulating layer is controlled by causing a silane compound having various substituents to chemically adsorb on the gate insulating layer. In this case, a uniform interface can be formed on an inorganic insulating layer made of SiO2 or the like, but it has been difficult to cause a silane compound to chemically adsorb to the surface of an organic insulating layer. In addition, Japanese Patent Application Laid-Open No. 2005-509299 discloses a field effect transistor in which a layer composed of polydimethylsiloxane is formed between a gate insulating layer and an organic semiconductor layer. However, no material allowing the formation of a uniform polymer layer on an organic insulating layer and the formation of an organic semiconductor layer on the polymer layer through application has been found. Further investigation into the obtainment of an optimum crystalline orientation with a view to increasing a carrier mobility is probably needed for the obtainment of stable properties of an organic semiconductor even on a flexible substrate.